Universal mobile telecommunication service (UMTS) systems refer to the 3rd generation asynchronous mobile communication system that is based on global system for mobile communications (GSM) and general packet radio service (GPRS) and uses wideband code division multiple access (WCDMA).
Presently, the 3rd generation partnership project (3GPP) standardizing UMTS is discussing long term evolution (LTE) as the next generation mobile communication system. LTE relates to a technology that implements fast packet based communication of the transmission rate, maximum 100 Mbps, and will be commercialized in 2010. To this end, many possible methods are under consideration, such as a method for simplifying the network structure and thus reducing the number of nodes located on the communication path, and a method for accessing radio protocol to a radio channel.
In an LTE mobile communication system, user equipment establishes a network and a radio resource control (RRC) connection. Establishing an RRC connection means establishing a signaling radio bearer that performs transmission and reception of control messages, such as an RRC control message, and the like. User equipment establishing an RRC connection is said to be in an RRC connection state.
The LTE mobile communication system applies discontinuous reception (DRX) to user equipment in an RRC connection state, thereby reducing power consumption. If DRX is established to user equipment in an RRC connection state, the user equipment monitors a forward control channel at a preset time point to check whether scheduling is performed. If scheduling has been performed, the user equipment performs data transmission/reception according to a procedure. Conversely, if scheduling has not been performed, the user equipment does not monitor the forward control channel until the next time point. These operations then are repeated.
Alternatively, if failures happen, user equipment in an RRC connection state starts an RRC connection reestablishment procedure. Examples of the failures are a radio link failure, a handover failure, and a failure to perform a command set by an ENB. Radio link failure refers to a case where, due to the aging of a channel in the user equipment, the user equipment cannot perform normal communication in a current cell. Handover failure refers to a case where user equipment cannot be moved to a cell where a handover of the user equipment is performed.
When an RRC connection reestablishment procedure starts, user equipment in an RRC connection state searches for and selects one of the cells, which provides adequate channel quality, and then transmits an RRC connection reestablishment request message to the selected cell. If the selected cell recognizes the user equipment, or contains information regarding the RRC connection of the user equipment, the ENB transmits an RRC connection reestablishment message to the user equipment and commands the user equipment to perform an RRC connection reestablishment. Conversely, if the selected cell does not recognize the corresponding user equipment, the ENB transmits an RRC connection reestablishment reject message to the user equipment and releases the RRC connection of the user equipment.
Presently, the LTE standard does not specify whether to retain discontinuous reception of user equipment or to change the discontinuous reception to new settings, when an RRC connection reestablishment procedure starts or a handover procedure starts in the user equipment in which a discontinuous reception was set.